[integ-test] Add instances to some tests in test definition

Similar change to #6508

Signed-off-by: Hanwen <hanwenli@amazon.com>

Author: hanwen-cluster

tests/integration-tests/configs/isolated_regions.yaml

@@ -80,6 +80,7 @@ test-suites:
         - regions: {{ REGIONS }}
           oss: {{ OSS }}
           schedulers: {{ SCHEDULERS }}
+          instances: ["t3.micro"]
     test_pcluster_configure.py::test_efa_and_placement_group:
       dimensions:
         - regions: {{ REGIONS }}